I'm having this problem and I know I blew my power valve recently. I have a 750 vacuum secondary on a stock 1971 460 Ford engine. When I'm driving it sometimes it will backfire and lose all power like its going to die. I hit the gas and it falls on its face but it will idle just fine. All of a sudden it will stop and ill have full power again. What could be causing these problems? Could it just be the power valve, accelerator pump, or maybe nozzle size?
If it shoots smoke from the tail pipe when the hesitation occurs go down a size in pump nozzle until its corrected. if no smoke comes out of tail pipe go up a squirt nozzle until its corrected. I agree with Jabba974 a 670 cfm is a little big for the motor. good luck.
First thing I would look at is timing...bigger cam needs more timing...then adjust the accelerator pump if need be... I wouldnt mess with the jets, a 670 cfm carb is pretty big on a 350ci...also make sure your idle screws are adjusted correctly... Working in a performance parts store for 15 years, i have only seen the need for a bigger carb a few times..
My 4160 vac sec does the same its driving me nuts!.I thought it was ignition so got a new 65,000 volt dist advanced my timing and re adjusted my air and fuel mixture screws. Doing this actually helped a little. Its not as bad but it still stumbles and runs like shit when I hit 4 grand only on the hwy. From a dead stop punching it there is the slight hesitation but after that it gets up to 55 grand really quick. I'm gonna do some more adjustments with my timing and carb and hope this goes away.
ok i have 670 street avenger and has 65 jets went up all the way to 79 and not much change . and at 4 grand all the way threw when hits 4 grand runs shitty .fine off start but when 65s in has stumble when punch it do i need 750 because plugs show it lean?
ok so i have 65 jets in front went up 14 sizes dont do much diffrent and pugs show it runs lean and at 4 grand all the way threw all them jets runs shitty when hits 4 grand .take it back down and have my dead spot at like 2 grand ? do i need buger carb like 670 to 750?
Hi,there.Check your orange throttle cam there should be 3 adjustment holes and they are numbered,if your engine idles at 600 to 700 rpm then put it to the #1 hole but if your engine idles over 1000 rpm then put it to the #2 or #3 hole it only takes a couple min.My chevy 355 motor had the same 670 street avenger holley carb, it stumbled and had a pretty bad dead spot just like yours.And my throttle cam was set at #2 so then i put it to #1 started up gave it throttle and it fixed the problem!!.
@cemusicman Ive tried just about every position on that cam, I actually bought a cam kit that has like 6 other cams..and tried a bunch of them too.. some helped in some ways, but just screwed things back up.. I think i went back to the orange or a tan cam.. I think I am having vacuum issues to tell the truth.
i have 350 BORED 30 WITH AIR GAP INTAKE 670 STREET AVENGER stumbles not sure on came any more did it 5 years ago and just got it into nova and have 3500 stall and falls on its face about that time .pisse me off any ideas?
I have the same problem with my Holley 4160 600cf in my Ski Nautique w/ 351 Windsor. Since im sitting right beside the engine while driving it underload its easy for me to see what the carb is doing. On mine the vacume sec was not opening. I tried a lighter spring, that didn't work. I pulled the check ball out of the vac sec and that did the trick. Seems like a shitty thing to do but it works.
I had alot of issue's with big mutha thumpr they need ALOT of initial timing I tried everything and at the end I put the vacuum hose from distributor to a full vacuum in the carb and backed off the timing just a little and everything went great and btw those cams needs alot of fuel so maybe you are running a little lean at idle check the nozzle and set the accelerator pump correctly tighten the bolt till there is a play and then open the bolts just a little to kill the play it should do it
I have a similar set up and I replaced the discharge nozzle (squirter) with a #31 and adjusted the bolt and spring with the cam that came with the carb, now mine is a 770 Street avenger but shouldn't mattter.
One other thing you may want to try is timing, these cams want Alot of initial (base) timing. I run an MSD Pro billet distributor now with a 6A box and Blaster SS coil with 24 degrees of base timing and total timing of 36 degrees. Runs better than 12-14 degrees base timing
I'm having this problem and I know I blew my power valve recently. I have a 750 vacuum secondary on a stock 1971 460 Ford engine. When I'm driving it sometimes it will backfire and lose all power like its going to die. I hit the gas and it falls on its face but it will idle just fine. All of a sudden it will stop and ill have full power again. What could be causing these problems? Could it just be the power valve, accelerator pump, or maybe nozzle size?
blue06spectra 2 months ago
its a timming issue
RMAHL 2 months ago
If it shoots smoke from the tail pipe when the hesitation occurs go down a size in pump nozzle until its corrected. if no smoke comes out of tail pipe go up a squirt nozzle until its corrected. I agree with Jabba974 a 670 cfm is a little big for the motor. good luck.
Silver240z 3 months ago
First thing I would look at is timing...bigger cam needs more timing...then adjust the accelerator pump if need be... I wouldnt mess with the jets, a 670 cfm carb is pretty big on a 350ci...also make sure your idle screws are adjusted correctly... Working in a performance parts store for 15 years, i have only seen the need for a bigger carb a few times..
jabba974 3 months ago
@jabba974 And you might want to look at the advance kit in that MSD dist..it comes with a pretty heavy spring in it out of the box... good luck
jabba974 3 months ago
My 4160 vac sec does the same its driving me nuts!.I thought it was ignition so got a new 65,000 volt dist advanced my timing and re adjusted my air and fuel mixture screws. Doing this actually helped a little. Its not as bad but it still stumbles and runs like shit when I hit 4 grand only on the hwy. From a dead stop punching it there is the slight hesitation but after that it gets up to 55 grand really quick. I'm gonna do some more adjustments with my timing and carb and hope this goes away.
MrRagingbull480 4 months ago
Oh ok well 50cc or better yet a pump cam on the primary lol i had the same isuse, check the vid on my page.. That was when I was first tuning it
stangredemption 5 months ago
ok i have 670 street avenger and has 65 jets went up all the way to 79 and not much change . and at 4 grand all the way threw when hits 4 grand runs shitty .fine off start but when 65s in has stumble when punch it do i need 750 because plugs show it lean?
calupjensen 5 months ago
ok so i have 65 jets in front went up 14 sizes dont do much diffrent and pugs show it runs lean and at 4 grand all the way threw all them jets runs shitty when hits 4 grand .take it back down and have my dead spot at like 2 grand ? do i need buger carb like 670 to 750?
calupjensen 5 months ago
I knoiw this is old but it's time for a 50cc accelerator pump or you need to adjust your pump arm or both if it's a double pumper
stangredemption 5 months ago
@stangredemption it is not a double pumper.
hectre13 5 months ago
yes they are but seems to stop at the stuble
calupjensen 6 months ago
did you look to make sure that fuel is coming out of the pump shooters?
ACHEESEDANISH00 6 months ago
@ACHEESEDANISH00 yup, they are shooting gas out, I actually replaced the nozzle with the next size up I believe and it seemed to help, but not much.
hectre13 5 months ago
@hectre13 yep 50cc is the only solution then.
ACHEESEDANISH00 5 months ago
Comment removed
ACHEESEDANISH00 6 months ago
Hi,there.Check your orange throttle cam there should be 3 adjustment holes and they are numbered,if your engine idles at 600 to 700 rpm then put it to the #1 hole but if your engine idles over 1000 rpm then put it to the #2 or #3 hole it only takes a couple min.My chevy 355 motor had the same 670 street avenger holley carb, it stumbled and had a pretty bad dead spot just like yours.And my throttle cam was set at #2 so then i put it to #1 started up gave it throttle and it fixed the problem!!.
cemusicman 7 months ago
@cemusicman Ive tried just about every position on that cam, I actually bought a cam kit that has like 6 other cams..and tried a bunch of them too.. some helped in some ways, but just screwed things back up.. I think i went back to the orange or a tan cam.. I think I am having vacuum issues to tell the truth.
hectre13 5 months ago
@hectre13 i would go double pumper. whats the specs on the thumper cam?
ACHEESEDANISH00 4 months ago
i have 350 BORED 30 WITH AIR GAP INTAKE 670 STREET AVENGER stumbles not sure on came any more did it 5 years ago and just got it into nova and have 3500 stall and falls on its face about that time .pisse me off any ideas?
calupjensen 7 months ago
I have the same problem with my Holley 4160 600cf in my Ski Nautique w/ 351 Windsor. Since im sitting right beside the engine while driving it underload its easy for me to see what the carb is doing. On mine the vacume sec was not opening. I tried a lighter spring, that didn't work. I pulled the check ball out of the vac sec and that did the trick. Seems like a shitty thing to do but it works.
SouthernStar2008 8 months ago
u should put a high volume fuel pump on it...
jpenn1192 1 year ago
@jpenn1192 it has a high volume fuel pump on it.. it was a bundle it came with the carb.. it is also a holley.
hectre13 5 months ago
Did you ever figure this out? I've got the same problem
aintmisbehavinn 1 year ago
@aintmisbehavinn not yet actually, its been sitting for a while because i am so frustrated with it.
hectre13 5 months ago
Does it ever back fire out of the carb or does it just stumble? Nozzle may be to big.
79fordblake 1 year ago
@79fordblake nope never has backfired.
hectre13 5 months ago
I just switched to a holley on my 351w, I'm having the exact same issue. My nozzle is a 25, i think I'm going to try a bigger nozzle first.
LtBullitt351 1 year ago
I had alot of issue's with big mutha thumpr they need ALOT of initial timing I tried everything and at the end I put the vacuum hose from distributor to a full vacuum in the carb and backed off the timing just a little and everything went great and btw those cams needs alot of fuel so maybe you are running a little lean at idle check the nozzle and set the accelerator pump correctly tighten the bolt till there is a play and then open the bolts just a little to kill the play it should do it
ThugLife383 1 year ago
I have a similar set up and I replaced the discharge nozzle (squirter) with a #31 and adjusted the bolt and spring with the cam that came with the carb, now mine is a 770 Street avenger but shouldn't mattter.
One other thing you may want to try is timing, these cams want Alot of initial (base) timing. I run an MSD Pro billet distributor now with a 6A box and Blaster SS coil with 24 degrees of base timing and total timing of 36 degrees. Runs better than 12-14 degrees base timing
carissawabo 1 year ago